High integrity CoCrAl(Y) coated nickel-base superalloys

ABSTRACT

A high temperature oxidation and corrosion resistant coated nickel-base superalloy article comprising (a) a nickel-base superalloy article, and .[.(b) a first.]. .Iadd.adjacent thereto (b) a .Iaddend.CoCrAl(Y) coating .Iadd.having a substantially uniform .Iaddend.composition .[.consisting essentially.]. .Iadd.composed .Iaddend.of, on a weight basis, approximately 26-32% chromium, 3-9% aluminum, 0-1% yttrium, the rare earth elements, platinum or rhodium, and the balance .[.nickel.]. .Iadd.cobalt and impurities ordinarily associated with the aforementioned constituents. .Iaddend.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a high temperature oxidation and corrosionresistant coated nickel-base superalloy article comprising (a) anickel-base superalloy article, and .[.(b) a first.]. .Iadd.adjacentthereto (b) a .Iaddend.CoCrAl(Y) coating .Iadd.having a substantiallyuniform .Iaddend.composition .[.consisting essentially.]. .Iadd.composed.Iaddend.of, on a weight basis, approximately 26-32% chromium, 3-9%aluminum, 0-1% yttrium, the rare earth elements, platinum or rhodium,and the balance .[.nickel.]. .Iadd.cobalt and impurities ordinarilyassociated with the aforementioned constituents.Iaddend.. An aluminideovercoating can be applied to the CoCrAl(Y) coated superalloys andconstitute another embodiment of my invention.

2. Description of the Prior Art

Evans et al. in U.S. Pat. No. 3,676,085 describe coated nickel-basesuperalloys wherein the coating composition consists essentially of, ona weight basis, 15-40% chromium, 10-25% aluminum, 0.01-5% yttrium or therare earth elements and the balance cobalt. Evans et al. .[.teache.]..Iadd.teach .Iaddend.that when the aluminum content of the CoCrAl(Y)coating is below about 10% there is insufficient aluminum present in thecoating system to provide the desired long term durability in thecoating.

Unexpectedly, I have found that nickel-base superalloys when coated withCoCrAl(Y) coatings having an aluminum content of less than 10% haveoutstanding physical and chemical properties, i.e. significant oxidationand corrosion resistance and high coating-substrate interface integrity.These outstanding properties are not associated with nickel-basesuperalloys when coated with the coating systems described by Evans etal. referenced above.

DESCRIPTION OF THE INVENTION

This invention embodies a high temperature oxidation and corrosionresistant coated nickel-base superalloy article comprising (a) anickel-base superalloy article of manufacture, and .[.(b) a first.]..Iadd.adjacent thereto (b) a .Iaddend.CoCrAl(Y) coating .Iadd.having asubstantially uniform .Iaddend.composition .[.consisting essentially.]..Iadd.composed .Iaddend.of, on a weight basis, approximately 26-32%chromium, 3-9% aluminum, and 0-1% yttrium, the rare earth elements,platinum or rhodium, and the balance .[.nickel.]. .Iadd.cobalt andimpurities ordinarily associated with the aforementionedconstituents.Iaddend..

My invention is more clearly understood from the following descriptiontaken in conjunction with the accompanying drawings, where:

FIG. 1 is a photomicrograph (500X) of a CoCrAl(Y) coated nickel-baseIN-738 superalloy having a coating composition of Evans et al. U.S. Pat.No. 3,676,085, i.e. Co-22Cr-13Al-1Y. This figure illustrates the lowintegrity of a CoCrAl(Y) coated nickel-base superalloy of the prior art,i.e. a coating which has a significant and a substantial tendency toseparate from a superalloy substrate thereby failing to give theoxidation and corrosion resistant coating integrity desired fornickel-base superalloys.

FIG. 2 is a photomicrograph (500X) of a CoCrAl(Y) coated nickel-baseIN-738 superalloy having a coating composition of my invention, i.e.Co-29Cr-6Al-1Y.

This figure illustrates the high integrity of a CoCrAl(Y) coatednickel-base superalloy of my invention, i.e. a coating .Iadd.ofsubstantially uniform composition .Iaddend.which does not have asignificant or substantial tendency to separate from a superalloysubstrate thereby giving the oxidation and corrosion resistance coatingintegrity desired for nickel-base superalloys. The coated nickel-basesuperalloys of my invention have a thermal expansion coefficient value αas measured in inches .Iadd.10⁻⁶ .Iaddend.per inch per °F. (in./in./°F.)over a temperature range of (i) 100°-1200° F. of 8.45 to 9.05in./in./°F. and (ii) 100° to 1740° F. of 9.45 to.Badd..[.10.5.]..Baddend. .Iadd.10.05 .Iaddend.in./in./°F.

A presently preferred nickel-base superalloy employed in my invention"IN-738" is of the following general composition:

    ______________________________________                                        Ingredient           IN-738                                                   ______________________________________                                        C                    0.17                                                     Mn                   0.10                                                     Si                   0.30                                                     Cr                   16.0                                                     Ni                   Bal.                                                     Co                   8.5                                                      Mo                   1.75                                                     W                    2.6                                                      Cb                   0.9                                                      Ti                   3.4                                                      Al                   3.4                                                      B                    0.01                                                     Zr                   0.10                                                     Fe                   0.50                                                     Other                1.75 Ta                                                  ______________________________________                                    

This superalloy has a thermal expansion coefficient value α measured in.Iadd.10⁻⁶ .Iaddend. in./in./°F. over the temperature ranges set outabove of 8.7±0.1 and 9.7±0.1, respectively.

A presently preferred CoCrAl(Y) coating employed in my invention "GT-29"is of the following general composition: Co-29Cr-6Al-1Y. This coatinghas a thermal expansion coefficient α measured in .Iadd.10⁻⁶ .Iaddend.in./in./°F. over the temperature ranges set out above of 8.8±0.1 and9.9±0.1, respectively.

.Iadd.As is established hereinbelow, a successful substrate/coatingcombination within the teachings of this invention employs IN-738 as thenickel-base superalloy and Co-29Cr-6Al-1Y as the coating composition.The approximate percent difference in coefficients of thermal expansionsuccessfully tolerated in the tests described are calculated from thedata set forth above as follows:

    __________________________________________________________________________    TEC*            MAX. TEC  %                                                   IN-738                                                                              Co-29Cr-6Al-1Y                                                                          DIFFERENCE                                                                              DIFFERENCE                                          __________________________________________________________________________    8.7 ± 0.1                                                                        8.8 ± 0.1                                                                             8.9 - 8.6 = 0.3                                                                         ##STR1##                                           9.7 ± 0.1                                                                        9.9 ± 0.1                                                                            10.0 - 9.6 = 0.4                                                                         ##STR2##                                           __________________________________________________________________________     *Thermal Expansion Coefficient 10.sup.-6 in./in./°F.              

Thus, in sum, this set of calculations shows that the combination ofthese materials had a maximum TEC difference of about 4 percent..Iaddend.

The nickel-base superalloys and CoCrAl(Y) alloys employed in myinvention can be prepared by any method well-known to those skilled.

The CoCrAl(Y) coatings can be applied to the nickel-base superalloys bymeans, such as physical or chemical vapor deposition, or any other meanswell-known to those skilled in the art for the application of CoCrAl(Y)coatings to superalloys. Among the coating techniques that can be usedare those described in

Flame Spray Handbook, Volume III, by H. S. Ingham and A. P. Shepard,published by Metco, Inc., Westbury, Long Island, New York (1965),

Vapor Deposition, edited by C. F. Powell, J. H. Oxley and J. M. Blocher,Jr., published by John Wiley & Sons, Inc., New York (1966), etc.

In general, the CoCrAl(Y) coated nickel-base superalloys can have anycoating thickness sufficient to give a desired oxidation and corrosionresistance. Generally economic and effective coating thicknesses are1-20 mils for most commercial applications. In preferred embodiments,where electron-beam techniques are employed the coating thicknessesrange from 1-5 mils and where plasma flame spray techniques are employedthe coating thicknesses range from 3-10 mils. In another preferredembodiment where an aluminide overcoating is employed, the aluminideovercoating--including any duplex heat treatment where the aluminideovercoating is heated for periods of time from 30 or 60 to 120 minutesat elevated temperatures of 850° to 1200° F. in air, argon, etc., forthe purpose of diffusing aluminum into the CoCrAl(Y) coating--the.[.aluminuide.]. .Iadd.aluminide .Iaddend.process is carried out in amanner which limits the aluminum penetration into the CoCrAl(Y) coatingto a distance no nearer than a 1/2 mil measured from the interface ofthe nickel-base superalloy and the CoCrAl(Y) coating. This aluminidediffusion penetration limitation is essential to the integrity of theCoCrAl(Y) nickel-base superalloy interface since as indicatedhereinbefore (as illustrated by FIG. 1) an increase in the aluminumcontent of the CoCrAl(Y) coating to levels of 10% or more deleteriousaffects the integrity of the coating composition.

My invention is further illustrated by the following examples:

EXAMPLE I

An experimental series was designed to study the expansion matchcharacteristics of nickel-base superalloys and CoCrAl(Y) compositions aswell as their oxidative and corrosion resistance.

Test specimen pins of IN-738 were prepared which had been lightlyabraded with a No. 3 alumina powder. The resulting pins were 4.4centimeters long and 0.25 cm. in diameter. A series of CoCrAl(Y) ingotshaving the compositions set out hereafter in Table I were electron-beamdeposited on the abraded IN-738 pin substrates at a deposition rate ofapproximately 0.1 mils per minute while the pins were rotated atapproximately 10 revolutions per minute. The coatings were deposited atvarious pins substrate temperatures, e.g. 1022° F., 1292° F., 1562° F.and 1832° F. The CoCrAl(Y) coated pins were thermal cycled duringdeposition over a temperature range of from approximately 1832° F. to70° F. (room temperature).

.[.Mellographic.]. .Iadd.Metallographic .Iaddend.examination viaphotomicrographs--illustrated by FIGS. 1 and 2--shows that the CoCrAl(Y)compositions of Evans et al. are not suited to nickel-base superalloysdefined herein since high aluminum CoCrAl(Y) coatings as deposited onthe nickel-base superalloy IN-738 separate from the substrate duringthermal cycling over a temperature range of from 1832°-70° F.

                  TABLE I                                                         ______________________________________                                                                      Associated                                                                    Photo-                                          Inventors        Compositions micrographs                                     ______________________________________                                        Evans et al., Prior Art                                                                            Co-18Cr-17Al-1Y                                                                            --                                          Compositions         Co-22Cr-13Al-1Y                                                                            FIG. 1                                                           Co-26Cr-9Al-1Y                                                                             --                                          Rairden's, This      Co-29Cr-6Al-1Y                                                                             FIG. 2                                      Invention's                                                                   Compositions (RD-7240)                                                                             Co-30Cr-9Al-1Y                                                                             --                                                               Co-32Cr-3Al-1Y                                           ______________________________________                                    

EXAMPLE II

Another series of CoCrAl(Y) coated IN-738 pin samples were prepared asdescribed in Example I above--having the coating compositions set outhereafter in Table II--were subjected to a burner rig test whichsimulated conditions used in a marine gas turbine engine under highlycorrosive conditions. The test was run to coating failure using a dieselfuel containing 1% by weight of sulfur and 467 parts per million seasalt at a temperature 1600° F. coupled with thermocycling to roomtemperature 3 to 5 times per week. The CoCrAl(Y) coated IN-738 sampleswere evaluated and characterized according to hours to failure, failurebeing defined as a condition wherein the results of the burner rigcorrosion test conditions set out in Table II hereafter:

                  TABLE II                                                        ______________________________________                                        Inventors    Compositions  Hours to Failure*                                  ______________________________________                                        Evans et al.     Co-22Cr-13Al-1Y                                                                              605                                           Rairden's        Co-32Cr-3Al-1Y                                                                              1235                                                            Co-29Cr-6Al-1Y                                                                              1675                                                            Co-30Cr-9Al-1Y                                                                              2431                                                            Co-26Cr-9Al-1Y                                                                              1594                                           ______________________________________                                         *Failure being defined as the approximate number of hours of test prior t     the formation of an observable bulky, green, nickelbearing oxide which        indicates that the coating has been penetrated under the burner rig test      conditions.                                                              

I claim:
 1. A high temperature oxidation and corrosion resistant coatednickel-base superalloy article .[.having a thermal expansion coefficientvalue in inches per inch per °F. measured over a temperature range of(i)100°-1200° F. of from 8.45 to 9.05, and (ii) 100°-1740° F. of from 9.45to 10.05;.]. .Iadd.characterized by high coating-substrate interfaceintegrity, said article .Iaddend.comprising: (a) a nickel-basesuperalloy, and (b) a .[.first.]. CoCrAl(Y) coating .Iadd.providing theouter surface of said article, said coating having a substantiallyuniform .Iaddend.composition .[.consisting essentially.]. .Iadd.composed.Iaddend.of, on a weight basis, approximately 26-32 percent chromium,3-9 percent aluminum, and 0-1 percent yttrium, other rare earthelements, platinum or rhodium, and the balance cobalt .Iadd.andimpurities ordinarily associated with the aforementionedconstituents.Iaddend..[...]..Iadd., said nickel-base superalloy and saidCoCrAl(Y) coating having substantially matching thermal expansioncoefficient values with the maximum difference between the thermalexpansion coefficient value of said substrate and the thermal expansioncoefficient value of said coating being about 4 percent over atemperature range from 100° F. to 1740° F. .Iaddend.
 2. A claim 1article, wherein.[.(a) said nickel-base superalloy consists essentiallyof, on a weight basis,.].

    ______________________________________                                        Ingredient           IN-738                                                   ______________________________________                                        C                    0.17                                                     Mn                   0.20                                                     Si                   0.30                                                     Cr                   16.0                                                     Ni                   Bal.                                                     Co                   8.5                                                      Mo                   1.75                                                     W                    2.6                                                      Cb                   0.9                                                      Ti                   3.4                                                      Al                   3.4                                                      B                    0.01                                                     Zr                   0.10                                                     Fe                   0.50                                                     Other                1.75 Ta                                                  ______________________________________                                    

.[.(b) said first.]. .Iadd.the .Iaddend.coating contains 29% chromium,6% aluminum, 1% yttrium and the balance cobalt. .[.3. A claim 2 article,further comprising (c) an overcoating of aluminum..]. .[.4. A claim 3article, wherein (b) said first coating has a thickness of about 1-20mils, (c) said second coating penetrates the first coating to a depth nonearer than 1/2 mil measured from the interface of the nickel-basesuperalloy and first coating..]. .[.5. A claim 4 article, wherein (b)said first coating is deposited by physical vapor deposition and thecoating thickness is about 1-5 mils, (c) said second coating isdeposited by chemical vapor deposition..]. .[.6. The claim 4 article,wherein (b) said first coating is deposited by plasma spraying and has acoating thickness of 3-10 mils..].